1. Field of the Invention
The present invention relates to a polishing method and polishing apparatus, a more particularly relates to a polishing method and polishing apparatus for flattening an uneven surface formed along with formation of copper interconnections in the process of production of a semiconductor device having copper interconnections.
2. Description of the Related Art
Along with the increase in integration and reduction of size of semiconductor devices, progress has been made in miniaturization of interconnections, reduction of interconnection pitch, and superposition of interconnections. The importance of the multilayer interconnection technology in the manufacturing process of semiconductor devices is therefore rising.
To form the interconnections of a semiconductor device of a multilayer interconnection structure, frequent use has been made of the method of forming the interconnections by aluminum or an alloy of the same, covering them by an insulating film, then burying contact holes passing through this insulating film by tungsten or an alloy of the same to connect with the lower interconnections.
With the above method, step differences arise due to the interconnections on the surface of the covering insulating film after the formation of the interconnections. Along with the miniaturization of interconnections, since the depth of focus at the photolithography of the upper layer can no longer be sufficiently matched with, a need has arisen for flattening these step differences. The method has been employed of polishing the insulating film by chemical mechanical polishing (CMP) method.
Further, use has been made of the metal CMP method of burying through holes formed in the insulating film by the above tungsten or other metal, removing the excess metal film by CMP, and thereby connecting with the lower interconnections.
On the other hand, in order to suppress the propagation delay of signals in the recent 0.25 xcexcm or less design rule, an interconnection process for replacing the aluminum of the interconnection material by copper is being developed. When using copper for interconnections, there is the merit that both a low resistance and a high electromigration tolerance can be obtained.
In a process using this copper for interconnections, for example, an interconnection process referred to as the damascene process for burying a metal in groove-like interconnection patterns formed in an interlayer insulating film in advance and removing excess metal film by CMP to form the interconnections has become dominant.
The damascene process has the characteristics that etching of the interconnections become unnecessary and also a further upper interlayer insulating film becomes flat by itself, so the process can be simplified.
Further, by the dual damascene process where not only grooves for the interconnections, but also the contact holes-are formed as grooves in the interlayer insulating film and where the interconnections and the contact holes are simultaneously buried by the metal, a greater reduction of the interconnection steps becomes possible.
Here, an explanation will be made of an example of the process for forming copper interconnections by the dual damascene process by referring to the figures below.
First, as shown in FIG. 11A, for example, an interlayer insulating film 302 made of for example silicon oxide is formed by for example low pressure chemical vapor deposition (CVD) on a silicon or other semiconductor substrate 301 on which a not illustrated impurity diffusion region is appropriately formed.
Next, as shown in FIG. 11B, contact holes CH communicating with the impurity diffusion region of the semiconductor substrate 301 and grooves M in which will be formed a predetermined pattern of interconnections to be electrically connected to the impurity diffusion region of the substrate 301 are formed by using well-known photolithography and etching.
Next, as shown in FIG. 1C, a barrier metal film 305 is formed on the surface of the interlayer insulating film 302 and in the contact holes CH and the grooves M. This barrier metal film 305 is formed by a material such as Ta, Ti, TaN, or TiN by well-known sputtering. When the interconnection material is copper and the interlayer insulating film 302 is silicon oxide, since copper has a large diffusion coefficient with respect to silicon oxide, it is easily oxidized. The barrier metal film 305 is provided to prevent this.
Next, as shown in FIG. 11D, a seed film 306 is formed on the barrier metal film 305 by depositing copper to a predetermined thickness by well-known sputtering.
Then, as shown in FIG. 11E, a copper film 307 is grown and formed on the seed film 306 so as to bury the contact holes CH and the grooves M by copper. The copper film 307 is formed by for example plating, CVD, sputtering, etc.
Next, as shown in FIG. 11F, the excess copper film 307 and barrier metal film 305 on the interlayer insulating film 302 are removed by CMP for flattening.
Due to the above steps, copper interconnections 308 and contacts 309 are formed.
By repeating the above process on the interconnections 308, multilayer interconnections can be formed.
Summarizing the problems to be solved by the invention, in the step of removing the excess copper film 307 by CMP in the copper interconnection forming process using the dual damascene process, because the flattening technique employing conventional CMP involves applying a predetermined pressure between a polishing tool and the copper film for polishing, there arises a problem that large damage is given to the semiconductor substrate. Especially, in a case where an insulating film of a small dielectric constant having a low mechanical strength, which will be important in the 0.13 xcexcm generation and on, is used, the aforesaid damage is no longer negligible and may cause cracks of the interlayer insulating film and separation of the interlayer insulating film from the semiconductor substrate.
Further, the removal performance differs among the interlayer insulating film 302, the copper film 307, and the barrier metal film 305, therefore there has been the problem that dishing, erosion (thinning), recess, etc. easily occur in the interconnections 308.
Dishing is a phenomenon where, as shown in FIG. 12, when there is an interconnection 308 having a width of, for example, about 100 xcexcm at a 0.18 xcexcm design rule, the center portion of the interconnection is excessively removed and sinks. If this dishing occurs, the sectional area of the interconnection 308 becomes insufficient. This causes poor interconnection resistance etc. This dishing is apt to occur when copper or aluminum, which are relatively soft, is used as the interconnection material.
Erosion is a phenomenon where, as shown in FIG. 13, a portion having a high pattern density such as where interconnections with a width of 1.0 xcexcm are formed at a density of 50% in a range of for example 3000 xcexcm is excessively removed. When erosion occurs, the sectional area of the interconnections becomes insufficient. This causes poor interconnection resistance etc.
Recess is a phenomenon where, as shown in FIG. 14, the interconnection 308 becomes lower in level at the interface between the interlayer insulating film 302 and the interconnection 308 resulting in a step difference. In this case as well, the sectional area of the interconnection becomes insufficient, causing poor interconnection resistance etc.
Further, in the step of flattening and removing the excess copper film 307 by CMP, it is necessary to efficiently remove the copper film. The amount removed per unit time, that is, the polishing rate, is required to be for example more than 500 nm/min.
In order to obtain this polishing rate, it is necessary to increase the polishing pressure on the wafer. When the polishing pressure is raised, as shown in FIG. 15, a scratch SC and chemical damage CD are apt to occur in the interconnection surface. In particular, they easily occur in soft copper. For this reason, they cause opening of the interconnections, short-circuiting, poor interconnection resistance, and other defects. Further, if the polishing pressure is raised, there is the inconvenience that the amount of the scratches, separation of interlayer insulating film, dishing, erosion, and recesses also becomes larger.
An object of the present invention is to provide a polishing method and a polishing apparatus capable of easily flattening unevenness formed on the surface of a film and able to efficiently polish the film flat while suppressing damage to an interlayer insulating film below a copper film when flattening the copper film or other film by polishing to form copper interconnections etc. in a semiconductor device.
To achieve the above object, according to a first aspect of the present invention, there is provided a polishing method for polishing a film of an object to be polished having a substrate, an insulating film formed in the substrate, interconnection grooves formed in the insulating film, and said film, that is, an interconnection layer, formed inside and outside of the interconnection grooves comprising supplying a processing solution over the surface to be polished at least substantially parallel to that surface and removing by polishing the film formed outside of the interconnection grooves by a shear stress due to the processing solution preferentially from projecting portions of the film to flatten the surface.
According to the polishing method of the present invention, when flattening a copper film or other film by polishing to form copper interconnections in a semiconductor device, the processing solution is supplied over the surface to be polished substantially parallel to that surface.
When the processing solution is supplied over the surface to be polished substantially parallel to that surface, the large kinetic energy processing solution directly strikes the side surfaces of projecting portions of any unevenness on the surface. On the other hand, the processing solution remains still in the recessed portions. The kinetic energy of the processing solution there is small. Further, the amount of the processing solution in contact and acting there itself is small. Therefore, the shear stress of the processing solution acts so that the projecting portions are processed faster than the recessed portions. The initial unevenness can therefore be easily flattened.
Further, due to the above action of the processing solution, the film can be polished efficiently while maintaining the flatness.
Further, since no strong pressure is applied to the surface as in chemical mechanical polishing, polishing is possible while suppressing the damage to the interlayer insulating film etc. below the film to be polished.
Preferably, the film comprises a copper film.
That is, the polishing method of the present invention is suitable as a method of polishing a copper film which had been processed by CMP in the conventional damascene process etc.
Preferably, the object to be polished is made an object having contact holes communicating with the interconnection grooves formed in the insulating film and having the interconnection layer formed buried inside the contact holes as well.
That is, the polishing method of the present invention is suitable as a method of polishing a copper film or other film which had been processed by CMP in a dual damascene process for forming contact plugs at the same time as interconnections in contact holes communicating with the interconnection grooves.
Preferably, the polishing method further comprises using a processing solution containing at least a chelating agent as the processing solution; chelating the surface part of the film by the chelating agent to form a chelate film; removing by polishing preferentially projecting portions of the chelate film by the shear stress of the processing solution; and repeatedly again forming a chelate film on the surface parts of the film exposed at the projecting portions and removing by polishing preferentially the projecting portions of the chelate film to flatten the film.
That is, the surface of the film is chelated by reacting with the chelating agent supplied as the processing solution whereby an easily removable low mechanical strength chelate film is formed. By supplying the processing solution over the surface formed with this chelate film so as to be substantially parallel to that surface, the large kinetic energy processing solution directly strikes the side surfaces of the projecting portions of any unevenness on the surface, whereby the chelate film at the projecting portions is easily and preferentially removed by polishing. New surfaces for polishing are exposed at the projecting portions from which the surface chelate film had been removed. These exposed surfaces are chelated again and struck by the processing solution to be preferentially removed by polishing. The film can therefore be flattened by repetition of this action.
More preferably, the polishing method further comprises using a polishing solution further including an oxidizing agent as the polishing solution and oxidizing the surface part of the film by the oxidizing agent and chelating the obtained oxide by the chelating agent to form a chelate film.
That is, the surface of the film to be polished is oxidized by reaction with the oxidizing agent. The oxidized surface of the film is then chelated by reaction with the chelating agent.
Alternatively, preferably the polishing method further comprises using a polishing solution further including a surface-active agent as the polishing solution and removing the chelate as micelles covered by the surface-active agent when removing by polishing from projecting portions of the chelate film by the shear stress by the processing solution.
If a surface-active agent is added to the processing solution in this way, the chelate film formed on the surface to be polished is converted to micelles by the surface-active agent. These micelles are not bound by any bonding force to the lower film and are merely precipitated and deposited there by their own weight, so can be easily removed by the processing solution flowing substantially parallel to the surface.
To achieve the above object, according to a second aspect of the present invention, there is provided a polishing method for an object having a film on a surface to be polished, comprising supplying an electrolytic solution at least between the surface to be polished and a cathode member arranged facing the surface and substantially parallel to the surface while supplying voltage with the cathode member as a cathode and the film as an anode to remove by polishing preferentially projecting portions of the film by the shear stress of the electrolytic solution to flatten the surface.
According to the polishing method of the present invention, when flattening an object having a film on the surface to be polished, the cathode member is arranged facing that surface and the electrolytic solution is supplied over that surface substantially parallel to that surface.
When voltage is supplied with the cathode member as a cathode and the film as an anode, the surface of the film is electrolytically eluted. Alternatively, when the electrolytic solution includes a chelating agent, due to this application of voltage, the surface of the film is anodically oxidized and is chelated by the chelating agent. At this time, since the electrolytic solution flows over the surface substantially parallel to the surface, a shear stress of the electrolytic solution acts to promote the electrolytic elution or removal of the chelate film. Due to this, the initial unevenness can be easily flattened. Further, the film can be polished efficiently while maintaining the flatness.
Further, since no strong pressure is applied to the surface as in chemical mechanical polishing, polishing is possible while suppressing the damage to the interlayer insulating film etc. below the film to be polished.
Preferably, the film comprises a copper-film.
That is, the polishing method of the present invention is suitable as a method of polishing a copper film which had been processed by CMP in the conventional damascene process etc.
Preferably, the polishing method further comprises using as the object to be polished an object having a substrate, an insulating film formed on the substrate, interconnection grooves formed on the insulating film, and said film, that is, an interconnection layer, buried inside the interconnection grooves and formed over the entire surface outside the interconnection grooves, and removing by polishing the film, that is, interconnection layer, formed outside of the interconnection grooves to flatten the surface.
More preferably, the object to be polished is made an object having contact holes communicating with the interconnection grooves formed in the insulating film and having the interconnection layer formed buried inside the contact holes as well.
That is, the polishing method of the present invention is suitable as a method of polishing a copper film or other film which had been processed by CMP in a dual damascene process for forming contact plugs at the same time as interconnections in contact holes communicating with the interconnection grooves.
Alternatively, preferably, the polishing method further comprises using an electrolytic solution containing at least a chelating agent as the electrolytic solution; supplying a voltage with the cathode member as a cathode and the film as an anode to oxidize the surface of the film by anodic oxidation; chelating the surface part of the oxidized film by the chelating agent to form a chelate film; removing by polishing preferentially projecting portions of the chelate film by the shear stress of the electrolytic solution; and repeatedly again forming a chelate film on the surface parts of the film exposed at the projecting portions and removing by polishing preferentially the projecting portions of the chelate film to flatten the interconnection layer.
Due to this, the oxidized surface of the film by anodic oxidation is chelated by reacting with the chelating agent supplied as the electrolytic solution whereby an easily removable low mechanical strength chelate film is formed. By supplying the electrolytic solution over the surface formed with this chelate film so as to be substantially parallel to that surface, the large kinetic energy electrolytic solution directly strikes the side surfaces of the projecting portions of any unevenness on the surface, whereby the chelate film at the projecting portions is easily and preferentially removed by polishing. New surfaces for polishing are exposed at the projecting portions from which the surface chelate film had been removed.
At this time, since the electrical resistance of the chelate film is far higher than that of the copper or other film, the film covered by the chelate film at the recessed portions remaining without being removed is inhibited from being anodically oxidized by the conductance. New chelate film is formed by anodic oxidation only at the projecting portions of the surface exposed by the removal of the chelate film.
Further, in the anodic oxidation, the shorter the distance between the cathode member and the film, the higher the current density, so even among the surfaces exposed by removal of the chelate film, the ones of the parts projecting out more have a larger current density and a faster speed of anodic oxidation so are accelerated in chelation.
By repeating this preferential removal of the projecting portions by the formation of a chelate film and striking action of the electrolytic solution with the chelate film, it is possible to flatten the film to be polished.
More preferably, the polishing method further comprises using an electrolytic solution further including a surface-active agent as the electrolytic solution and removing the chelate as micelles covered by the surface-active agent when removing by polishing from projecting portions of the chelate film by the shear stress by the electrolytic solution.
If a surface-active agent is added to the electrolytic solution in this way, the chelate film formed on the surface to be polished is converted to micelles by the surface-active agent. These micelles can be easily removed by the electrolytic solution flowing substantially parallel to the surface.
To achieve the above object, according to a third aspect of the present invention, there is provided a polishing apparatus for polishing an object having a film on a surface to be polished, comprising a processing solution supplying means for supplying a processing solution over the surface to be polished at least substantially parallel to that surface and removing by polishing the film formed outside of the interconnection grooves by a shear stress due to the processing solution preferentially from projecting portions of the film to flatten the surface.
When flattening the copper film or other film by polishing to form copper interconnections in a semiconductor device, the apparatus supplies the processing solution over the surface to be polished substantially parallel to that surface. The large kinetic energy processing solution directly strikes the side surfaces of projecting portions of any unevenness on the surface. On the other hand, the processing solution remains still in the recessed portions. The kinetic energy of the processing solution there is small. Further, the amount of the processing solution in contact and acting there itself is small. Therefore, the shear stress of the processing solution acts so that the projecting portions are processed faster than the recessed portions. The initial unevenness can therefore be easily flattened.
Further, due to the above action of the processing solution, the film can be polished efficiently while maintaining the flatness.
Further, since no strong pressure is applied to the surface as in chemical mechanical polishing, polishing is possible while suppressing the damage to the interlayer insulating film etc. below the film to be polished.
Preferably, the polishing apparatus polishes an object wherein the film comprises a copper film.
That is, the polishing apparatus of the present invention is suitable as an apparatus for polishing a copper film which had been processed by CMP in the conventional damascene process etc.
Preferably, the polishing apparatus supplies a processing solution containing at least a chelating agent from the processing solution supplying means.
More preferably, it supplies a processing solution further containing an oxidizing agent from the processing solution supplying means.
Alternatively, it supplies a processing solution further containing a surface-active agent from the processing solution supplying means.
Due to this, the surface of the film oxidized by the oxidizing agent etc. is chelated by reacting with the chelating agent supplied as the processing solution, whereby an easily removable low mechanical strength chelate film is formed. This can be easily removed by the processing solution supplied over the surface so as to be substantially parallel to that surface, whereby the film can be flattened.
If a surface-active agent is added to the processing solution in this way, the chelate film formed on the surface to be polished is converted to micelles by the surface-active agent. These micelles are not bound by any bonding force to the lower film and are merely precipitated and deposited there by their own weight, so can be easily removed by the processing solution flowing substantially parallel to the surface.
To achieve the above object, according to a fourth aspect of the present invention, there is provided a polishing apparatus for an object having a film on a surface to be polished, comprising a cathode member arranged facing the surface; an electrolytic solution supplying means for supplying an electrolytic solution between the surface and the cathode member and over the surface at least substantially parallel to the surface; and a power supply for supplying voltage with the cathode member as a cathode and the film as an anode and removing by polishing preferentially projecting portions of the film by the shear stress of the processing solution.
When voltage is supplied with the cathode member as a cathode and the film as an anode, the surface of the film is electrolytically eluted. Alternatively, when the electrolytic solution includes a chelating agent, due to this application of voltage, the surface of the film is anodically oxidized and is chelated by the chelating agent. At this time, since the electrolytic solution flows over the surface so as to be substantially parallel to the surface, a shear stress of the electrolytic solution acts to promote the electrolytic elution or removal of the chelate film. Due to this, the initial unevenness can be easily flattened. Further, the film can be polished efficiently while maintaining the flatness.
Further, since no strong pressure is applied to the surface as in chemical mechanical polishing, polishing is possible while suppressing the damage to the interlayer insulating film etc. below the film to be polished.
Preferably, the polishing apparatus polishes an object wherein the film comprises a copper film.
That is, the polishing apparatus of the present invention is suitable as an apparatus for polishing a copper film which had been processed by CMP in the conventional damascene process etc.
Preferably, the polishing apparatus supplies an electrolytic solution containing at least a chelating agent from an electrolytic solution supplying means.
More preferably, it supplies an electrolytic solution further containing a surface-active agent from the processing solution supplying means.
Due to this, the anodically oxidized surface of the film is chelated by reacting with the chelating agent supplied as the electrolytic solution, whereby an easily removable low mechanical strength chelate film is formed. Due to the electrolytic solution flowing substantially parallel to that surface, the chelate film at the projecting portions is easily and preferentially removed by polishing. New surfaces for polishing are exposed at the projecting portions from which the surface chelate film had been removed.
At this time, since the electrical resistance of the chelate film is far higher than that of the copper or other film, the film covered by the chelate film at the recessed portions remaining without being removed is inhibited from being anodically oxidized by the conductance. New chelate film is formed by anodic oxidation only at the projecting portions of the surface exposed by the removal of the chelate film.
Further, in the anodic oxidation, the shorter the distance between the cathode member and the film, the higher the current density, so even among the surfaces exposed by removal of the chelate film, the ones of the parts projecting out more have a larger current density and a faster speed of anodic oxidation so are accelerated in chelation.
By repeating this preferential removal of the projecting portions by the formation of a chelate film and striking action of the electrolytic solution with the chelate film, it is possible to flatten the film to be polished.
If a surface-active agent is added, the chelate film formed on the surface to be polished is converted to micelles by the surface-active agent. These micelles can be easily removed by the processing solution flowing substantially parallel to the surface.
Preferably, the power supply is a direct current power supply which supplies a predetermined voltage with the cathode member as a cathode and the film as an anode.
More preferably, the direct current power supply supplies a pulse-like voltage having a predetermined period.
For example, making the pulse width extremely short makes the amount of formation of chelate film by the anodic oxidation per pulse very small. This is effective for preventing sudden and huge anode oxidation of the copper film caused by discharge due to a sudden change of the distance between electrodes in a case of contact with unevenness of the surface or spark discharge due to a sudden change of electrical resistance occurring when air bubbles, particles, or the like are interposed and for achieving continuity of as small amounts as possible.
Preferably, the apparatus further comprises an anode member facing the surface to be polished and separated from the cathode member by a predetermined distance; the electrolytic solution supplying means supplies an electrolytic solution between the surface and the cathode member and between the surface and the anode member; and the power supply supplies voltage to the cathode member and the anode member.
More preferably, the anode member is comprised of a nobler metal than the material of the film.
Due to this, it possible to prevent elution of the cathode member into the electrolytic solution and to positively cause anodic oxidation of the copper film. Note that since the cathode is not eluted, there is no need to consider the relative nobleness of the material.
Preferably, the polishing apparatus further comprises an ammeter for measuring the value of a current flowing between the cathode member and the film.
More preferably, it further comprises a controller for controlling the voltage supplied by the power supply so that the current value obtained from the ammeter becomes constant.
By controlling the current value to be constant, the current density becomes constant at all times and the amount of formation of chelate film by the anodic oxidation can be controlled to be constant.
Further, by monitoring the electrolytic current, it is possible to manage the polishing process and possible to obtain an accurate grasp of the state of progress of the polishing process.
More preferably, the polishing apparatus manages progress of polishing of the film by a direct current value obtained from the ammeter.
For example, by using a chelating agent giving an electrical resistance of the chelate film formed larger than a copper film, the current flowing between the cathode member and copper film removes the chelate film of the projecting portions before the projecting portions of the copper film are flattened. Since the copper is then exposed, the current value increases and new chelate film is formed on the exposed copper, whereupon the current falls. This pattern is repeated. On the other hand, when the copper film is flattened, the entire surface of the copper film is exposed by the removal of the chelate film from all of the copper film, so the current value reaches a maximum value. The current value reaches its maximum level with each subsequent removal.
Here, when all of the copper film on the barrier metal film at the outside of the interconnection grooves is removed and the barrier metal film is exposed, since the resistance of the barrier metal film is usually larger than that of copper, the current value starts to fall from its maximum level after the removal of the chelate film, so by stopping the application of voltage when it starts to fall, it is possible to stop the formation of the chelate film by subsequent anodic oxidation and therefore possible to manage the progress of polishing.